The measurement of pH (log 1/[hydrogen ion concentration]) of a material is a common and routine analytical activity used in a variety of industries. A commonly used device has a pH electrode and a reference electrode, which, when contacted with the sample, comprise an electrochemical cell. The electromotive force so generated by this cell may be calibrated against the pH of the sample.
The principal limitation of this approach is that these devices are not suited to operation under extreme conditions of pressure (up to and exceeding 1000 MPa). In addition to mechanical strength considerations; the reaction equilibria (as well as the diffusive mass transport necessary for achievement of such equilibria) are altered in complex ways by pressure, resulting in uncertainty regarding changes in sensor response under these extreme conditions. The feed-through wires used in pressurized experimental set-ups are prone to electrical interference, resulting in noisy signals that obscure the millivolt-level readings of such sensors. Experiments with such sensors (iridium-iridium oxide sensing electrode and silver-silver-chloride reference electrode, for example) have suggested that reproducibility is a major issue—sensors typically take up to eight cycles of pressurization before consistent results are obtained. Further, such sensors break down frequently.
Methods described for determination of pH at high pressures typically use data on reaction volumes under atmospheric pressure, and using thermodynamic considerations to calculate pH under pressure. These methods represent calculations that have not been experimentally verified. Still another method measures optical properties of transparent solutions and relates these to pH. However, such a method is not applicable to most real materials, which are opaque.
The pressure affects reaction equilibria and conformation of molecules leading to pH changes under high pressure. The measurement of pH under pressure has been investigated over the past several decades. Disteche (1959) made a glass pH electrode capable to withstand high hydrostatic pressure up to 1500 kg/cm2 (147 MPa). The electrode assembly also consisted of two Ag/AgCl electrodes and was designed to use pH measurements at great ocean depth. Crolet and Bonis (1983 and 1984) also used a glass pH electrode with a Ag/AgCl reference electrode to measure pH under pressure from 0.1 up to 100 MPa. The fragile nature of glass pH electrode however limits its use under high pressures above 150 MPa. In addition, it is reported that the electrode potential of the Ag/AgCl reference electrode is pressure dependent (Cruanes et al., 1992), which makes the pH measurement more complicated under pressure.
High pressure pH measurements (up to 450 MPa) were also performed using optical methods (Hitchens and Bryant, 1998; Hayert et al., 1999; Stippl et al., 2002). These methods involve indicator dyes and are not suitable for opaque samples such as food.
In addition to the above in-situ methods, pH of buffer solutions has been calculated using the electrical conductivities measured under high pressure (Kalinina and Kryukov, 1976). This is however not a direct method of measuring pH and is not applicable for complex samples such as food. The pH of buffer solutions under high pressure was also calculated using the experimental values of reaction volume under atmospheric pressure (El'yanov, 1975; Kitamura and Itoh, 1987). Their theoretical models assume reaction volume is pressure independent, which may not be true in reality.